For the quantitative determination of a sample substance, conventional electroanalysis using electrolysis involves measuring electrical changes occurring in a reaction (e.g., oxidation, reduction, etc.) of the substance under application of constant voltage or current, and in recent years, electrically conductive porous electrodes have been applied to such analysis.
Conventional electroanalysis using ion electrodes is a method in which an ion passing through a separating membrane is detected and in which an equilibrium potential between ions resulting from a sample substance and an electrode membrane is electrically measured by each electrode immersed as a detector in a sample solution. Another conventional electroanalysis using conductive porous electrodes is a method in which a sample is directly electrolyzed in a detection chamber with a conductive porous substance impregnated with an electrolyte, as described e.g. in Japanese Published Unexamined Patent Application 195,358/89. According to this patent application, however, electrical energy should be supplied from the outside, and a long time is required for the stabilization of the measurement device.
Conventional analysis by a reaction via a mediator makes use of a reaction e.g. from nicotinamide adenine dinucleotide (NAD) to reduced nicotinamide adenine dinucleotide (NADH) by dehydrogenase. For such a method, a glutamic acid assay kit (manufactured by BMY Co., Ltd.), for example, is commercially available for the quantitative determination in colorimetry of glutamic acid by glutamate dehydrogenase.
However, the aforesaid electroanalysis by ion electrodes have problems such as a long measurement time due to a slow rate of ion transfer through a separating membrane and a slow response rate of reaching equilibrium, etc., and troublesome procedures such as the step of washing required after each measurement, etc.
The above electroanalysis by electrolysis also has a problem. Electrolysis of a sample substance needs constant voltage or current from an external power source. Such application of voltage, however, causes the electrodes to be in a nonequilibrium state where so-called "capacitative current" occurs. Therefore, every time voltage is applied, a long stabilization time is required for measurement by electrolysis. Another problem is that the device should be made complex because of the necessity of constant voltage or current generating units, etc.
Furthermore, the above-mentioned measurement by enzyme reaction via a mediator requires the troublesome procedures of forming e.g. NADH with an enzyme, then reacting it with diaphorase etc., and measuring its absorbance. There are also practical problems: e.g., a refrigerator is required for storage of enzyme etc., and a sensor intended for use at room temperature cannot used for enzyme. In addition, it is not possible to accurately determine the absorbance where a colored substance is present in the sample.